Scattering Filter Improves Organic LED

Michael A. Greenwood

Researchers at MIT in Cambridge, Mass., have created an efficient and saturated deep-blue color by coupling a strong microcavity organic LED with a scattering filter.

The technique allowed phosphorescent OLEDs to overcome angular dependence — where the color and intensity depend on the position of the viewer — associated with strong microcavities. It also outperformed conventional structures in terms of how efficiently light was produced.

This image shows a schematic of a microcavity OLED with an external holographic diffuser. In the absence of the microcavity, the OLED emits a blue-green color. The microcavity is used to select blue emission. The diffuser corrects the angular dependence of emission from the microcavity.

Attaining usable blues for display applications with phosphorescence has typically been difficult because the color is diluted by longer wavelengths and loses its saturation, said lead researcher Marc A. Baldo. The researchers overcame the problem by placing the sky-blue phosphor FIrpic into a strong microcavity between two highly reflective mirrors, thus influencing the wavelengths of the emitted light. The scattering filter corrected the light’s angular intensity and color shifting.

They tested three types of filters — frosted glass, opal glass and holographic diffusers — and found that the holographic diffuser achieved an optical transparency close to 100 percent. The other two methods achieved results well below that figure.

The holographic diffuser also attained a peak quantum efficiency of 5.5 ±0.6 percent. A conventional weak microcavity structure, by comparison, achieved an efficiency of 3.8 ±0.4 percent. The average color coordinates (X,Y) were 0.116 ±0.004, 0.136 ±0.010.

The scientists said the technique could be used to create OLEDs suitable for display applications. The same method — a strong microcavity used in conjunction with the appropriate scattering filter — also could be used to fine-tune red, green and blue pixels in three-color display applications.

A disadvantage of the technique is the complexity of the architecture involved. Also, the thickness of the organic layers in the optical cavity must be precisely controlled, Baldo said.

The researchers suggest that saturated blue OLEDs with efficiencies higher than 10 percent are attainable.

Applied Physics Letters, May 21, 2007, 211109.